1. Field of the Invention
The present invention relates to the field of photodynamic therapy and, in particular, to a device and method for controlled delivery of light to the nasopharyngeal cavity for the treatment of cancer.
2. Information Disclosure Statement
Photodynamic therapy (PDT) is a minimally invasive, non-surgical treatment option for a variety of diseases, including cancer. PDT utilizes photosensitizing drugs and specific wavelengths of visible light to generate singlet oxygen, which in turn, induces oxidative damage in target tissues. The cumulative effects of oxidative damage, namely necrosis, apoptosis, and/or vascular collapse, result in the localized destruction of the target tissue. PDT treatment results in localized and specific tissue destruction, in part, because visible light has a tissue penetration range of between 2.0-6.0 mm. As a result, PDT treatment substantially reduces trauma to underlying, healthy tissues and organs. In light of its localized and selective biological effects, PDT is an attractive alternative to other tumour therapies, such as chemotherapy and radiotherapy, especially when targeting cancerous tissues in the nasopharynx.
Conventional treatment of nasopharyngeal cancer consists of different combinations of chemotherapy and radiotherapy dosing regimes, which require high dose and high-precision techniques such as brachytherapy and stereotactic radiotherapy. These techniques deliver a maximum possible radiation dose to the nasopharyngeal cavity to limit damage to critical tissues such as the spinal cord, the optic nerve and the optical chiasma. However, this treatment scheme is time-consuming, technically demanding, and very stressful to the patient. Conventional radiotherapy methods often cause permanent radiation damage and unpleasant, long-term side effects. For example, xerostomia (dry mouth) is the result of permanent radiation damage to the salivary glands. Moreover, because conventional techniques administer a maximum radiation dose to the nasopharyngeal area, repeat dosing of radiotherapeutics is not an option in the event of recurrence. For recurring disease, surgery is the only treatment method currently available but it is rarely curative and highly disfiguring.
For patients experiencing recurring disease in the nasopharynx post-radiotherapy, PDT would be an excellent alternative to surgery. Unlike conventional radiotherapy methods, PDT could provide effective treatment of superficial (surface) tumours without substantial risk to adjacent healthy tissues and organs. In addition, using PDT as a local booster could either supplement conventional radiotherapy or, in certain situations, completely supplant radiotherapy altogether.
For effective destruction of target tissues, PDT requires uniform irradiation of the diseased tissue in the proper amount. Consequently, light intensity and light dose are critical factors to the overall efficacy of PDT treatments. But, PDT treatment in the nasopharyngeal cavity is problematic for two reasons: (1) light scattering and (2) the irregular geometry of the nasopharyngeal cavity. These two factors hamper the consistent delivery of a therapeutically effective light dose to the target tissue, which reduces the overall efficacy of PDT treatment.
The interaction of light with biological tissue is a complicated process that changes with tissue type, fluence rate, delivered dose, and wavelength of light. The processes of reflection, scattering, and partial absorption, diffuses light as it penetrates into biological tissue. Light dosimetry in PDT treatments is complicated by the light-tissue interaction process, which effectively increases the local light fluence rates and causing target tissues to receive a light dose that is many times higher than the original incoming light alone.
In an effort to deliver more uniform irradiation in hollow cavities such as, the bladder, the oesophagus, the oral and thoracic cavities, and the bronchi, a few dedicated light delivery systems have been developed for PDT treatment at individual treatment sites. In particular, Farr et al, in U.S. Pat. No. 5,997,571, titled, Non-Occluding Phototherapy Probe Stabilizers,” describe a device for PDT treatments within the bronchia of the lung or other lumens. These researchers teach a fiber optic apparatus with a series of “non-occluding expansion elements” that can be “activated to protrude from the catheter body and contact the lumen walls” for centering an optical fiber. The disclosed projection elements are either rigid or inflatable. This reference does not teach a device expressly dedicated to PDT treatment in the nasopharyngeal cavity, nor does the reference predict or teach the successful use of the disclosed apparatus in the nasopharynx.
Lundahl in U.S. Pat. No. 4,998,930, entitled, “Intracavity Laser Phototherapy Method,” discloses another irradiation method especially suited for irradiating inside the human urinary bladder. Lundahl teaches a balloon catheter for centering a light source within the bladder prior to laser therapy. The disclosure further teaches a method of positioning a fiber optic light source in an irregularly shaped cavity that requires inflation of a balloon. Inflation of the balloon reshapes the cavity into a relatively uniform spherical shape thus allowing for uniform irradiation of tissue.
The Rotterdam Nasopharynx Applicator (RNA) developed by Levendag et al. for positioning catheters to deliver radioactive materials into the nasopharyngeal cavity in brachytherapy is discussed in Radiotherapy and Oncology, 1997; pp. 95-88. Specifically, the RNA was designed to accommodate the use of fractionated high dose radiation protocols on an out-patient basis. To accommodate the requirements of out-patient brachytherapy, the RNA was designed to remain in situ for the duration of the radiotherapy treatment which varies from 2 to 6 days. However, due to the particular nature and methods of radiation therapy, the RNA is not particularly suited for use in PDT treatment of the nasopharynx.
The above-described references disclose devices/methods optimized for the treatment of spatially straightforward cavities, i.e., closer to a spherical or a cylindrical shape. For spatially straightforward surfaces, the fluence rate distribution at any point on the irradiated surface can be accurately predicted by measuring the fluence rate at a few select points. However, the nasopharynx is more spatially complex than either the spherically shaped bladder or the cylindrically shaped bronchi. This spatial complexity makes extrapolating the fluence rate distribution at any point in the nasopharynx from a small number of point measurements problematic at best. Moreover, due to the irregular geometry of the nasopharyngeal cavity, irradiation methods and devices which are suitable for spherical or cylindrical cavities, do not offer stable and reproducible positioning in the nasopharynx together with uniform irradiation of nasopharyngeal target tissues. Thus, PDT treatments in the nasopharynx require a very different approach to choosing illumination parameters.
As discussed above, the main setback for photodynamic therapy in the nasopharynx is the lack of a convenient, reproducible and controlled method for delivering light to the nasopharyngeal cavity. Treatment within the nasopharyngeal cavity necessitates the identification of“at risk” tissue areas as well as the treatment target area, which are determined beforehand using MR images or CT scans. This process is similar to the Planning Risk Volume (“PRV”) and Planning Target Volume (“PTV”) concepts used in radiotherapy planning. Using prior clinical experience, the minimum dose for delivery to the PTV and maximum dose for the PRV is chosen. However, uniform irradiation is critical for preventing under-exposure of diseased areas, as this would lead to recurrence of the tumour. Equally important is to prevent over-exposure of healthy or delicate tissues whose damage would lead to serious complications for the patient. The uniform illumination of the nasopharyngeal cavity thus requires a specialized device for use in PDT treatment in this area.
The present device and method address this need.